Positive displacement motors (motors) are well known in the art, and are primarily used to drive drill bits in directional drilling motors. Such motors are colloquially known as “mud motors” as they rely on a pressurized flow of drilling mud or fluid to drive them. Such motors operate pursuant to the Moineau principle and are also known as progressive cavity motors. The power section of a positive displacement motor (motor) converts the hydraulic energy of high pressure drilling fluid to mechanical energy in the form of torque output for the drill bit. A power section consists of a helical-shaped rotor and stator. The rotor has a number of helical lobes, and is typically made of steel and is either chrome plated or coated for wear resistance. The stator is a heat-treated steel tube lined with a helical-shaped elastomeric insert. The rotors have one less lobe than the stators and when the two are assembled, a series of cavities is formed along the helical curve of the power section. Each of the cavities is sealed from adjacent cavities by seal lines formed along the contact line between the rotor and stator, which are critical to power section performance.
High pressure fluid is pumped into one end of the power section, where it fills the first set of open cavities. The pressure differential across two different cavities causes the rotor to turn. This filling and rotation process repeats in a continuous manner as long as high pressure fluid is being delivered to the power section.
Slip is caused when high pressure fluid blows by the rotor and stator seal lines, resulting in power section speed reduction. During downhole operation, differential pressure and slip increase and the load on bit increases. Many factors affect slip, and finding an optimal fit between rotor and stator is critical to balance stator life and slip efficiency. Power section failures are primarily due to destruction of the stator elastomer.
A typical positive displacement motor requires a large volume of high pressure fluid, and is therefore very inefficient if used in a production setting, as opposed to a drilling operation.